Alloys made from aluminum and lithium are very useful since they are lightweight and of high strength. The aerospace industry is especially interested in such alloys because of these features, and their employment in an airframe can reduce the weight of the structure by about 10%.
Precise thermal and thermal-mechanical treatments are required when manufacturing these alloys, however, in order to ensure that the required material properties are obtained. During the precipitation of such alloys, several crystal phases may occur. There is, for example, the aluminum-lithium solid solution phase, and also the metastable cubic stage comprised of Al.sub.3 Li having a superlattice structure. The latter cubic phase is highly desirable in that it is through this structure that the alloy gains its strength. The equilibrium phase, T.sub.1, on the other hand has a hexagonal structure. The T.sub.1 phase can increase the yield strength of the alloys, but has detrimental effects encouraging fatigue crack propagation. It is desired that this phase (T.sub.1) be avoided. The typical precipitation sequence for an aluminum-lithium alloy containing between 2% and 5% copper, for example, advances from the solid solution phase to the cubic phase, and further to a cubic phase mixed with the T.sub.1 phase.
Strong interest has developed in research on the use of sensors for monitoring the state of the material during processing and in service. In this regard, conductivity measurements and hardness measurements have been found to be particularly sensitive to the changes occurring in the aging of aluminum alloys. See, e.g., L. J. Swartzendruber, W. J. Boettinger, L. K. Ives, S. R. Coriel, and R. Mehrabien, "Nondestructive Evaluation: Microstructural Characterization and Reliability Strategies", O. Buck and S. M. Wolf, eds. TMS-AIME, Warrendale, Pa. 1981, p. 253. Electrical conductivity, in general, increases with aging. These measurements have been used to determine hardness and conductivity of alloys and to monitor condition of the alloy. Computer devices have been developed in order to read eddy current probing of an alloy which is one method of measuring conductivity. E.g., Howard, U.S. Pat. No. 4,450,405. The hardness and the eddy current increase in direct correlation to one another. Thus, researchers to date have used these measurements in order to find the correlating increase in hardness and eddy current profile.
To date, none of these known methods have been found useful in determining the presence of the undesired T.sub.1 phase in aluminum-lithium alloys, and there is a need for such a T.sub.1 detection system. This invention results from the surprising discovery that the T.sub.1 phase may be detected by the dramatic deviation in hardness measurements versus conductivity measurements occurring when that T.sub.1 phase is present.
Additionally, detecting such a T.sub.1 phase without destruction of the material is especially useful since it can be used on alloys already in use, or for those on the production line.
Accordingly, it is an object of the invention to provide for a method of testing aluminum-lithium alloys to detect the possibility of cracking or similar defects.
It is a further object of the present invention to provide for a method of detecting T.sub.1 phase in aluminum-lithium alloys.
It is another object of this invention to provide for a method of detecting T.sub.1 phase in aluminum-lithium alloys that does not destroy the alloy.
A further object of the invention is to provide for a convenient and inexpensive method of detecting T.sub.1 phase in aluminum-lithium alloys.
Yet another object of the invention is to provide for a method of detecting T.sub.1 phase in aluminum-lithium alloys that may be used on alloys during production or when such alloys are in use.